TMJ is an acronym for temporomandibular joint disorder. Often an extremely painful condition, it is caused by displacement of the cartilage where the lower jaw connects to the skull. This is one of the most commonly used joints in the body. (It moves every time you chew, talk or use your mouth at all.) The displacement creates a painful pressure and stretching of the associated sensory nerves. You might have TMJ if you feel like your jaw is locking or clicking, if you have a problem opening your mouth fully or if you have frequent headaches or pains in your neck.  Not all causes are known. Some possible causes or contributing factors are injuries to the jaw area, various forms of Arthritis, dental procedures, genetics, hormones, low-level infections, auto-immune diseases, stretching of the jaw as occurs with inserting a breathing tube before surgery, and clenching or grinding of the teeth.